Thinking about the implications of bit stuffing I wrote about in the SDLC post, I realized that long sequences of ones would be transmitted slower than long sequences of zeroes due to an extra bit being inserted after every fifth consecutive one. The theory would predict a 20% decrease in transmission speed.

Of course I wanted to test this phenomenon immediately. I connected two routers with a low-speed (64 kbps) link, and started a series of pings. Not surprisingly, the results confirmed the theory:

Speaking of Joe Harris … he just wrote a great post about the IP routing table profiling available in Cisco IOS and the ways it can be used to monitor the stability of your network.

Joe Harris published an excellent post detailing how you can use Flexible Packet Matching to recognize (and potentially block) Skype traffic. The solution depends on recognizing the first four bytes sent by the Skype application in a TCP session. While this is a great idea, you have to be aware that there's always a non-zero chance of false positives, more so as the described filter is testing the beginning of the payload in every TCP packet (not just the first data packet in the session).

My recent post about problems with old modems has generated a lot of comments with some very useful ideas, but nobody addressed the question “why was a long string of ones not a problem?”, so let's start there. Almost all WAN synchronous protocols in use today are descendants of venerable SDLC invented by IBM more than 30 years ago.

SDLC was later extended to support connectionless and balanced modes, resulting in HDLC. PPP is just an extension of HDLC, adding support for negotiations and standard layer-3 protocol demultiplexing.

William Chu sent me a working configuration he uses to measure jitter with the IP SLA tool and react to excessive jitter on the primary link. First you have to create the jitter probe with the IP SLA commands:

ip sla monitor 3000
 type jitter →
   dest-ipaddr 199.11.18.168 dest-port 12333 →
   source-ipaddr 199.11.18.169 codec g729a →
   codec-numpackets 100
 tos 184
 frequency 10

Note: The continuation character (→) indicates that the configuration command spans multiple lines

In the pre-DSL days, you had two options to get a short-haul high-speed link (at least in Europe): take E1 (or fractional E1) from a telecom (which was more expensive than a highway robbery, as the cost was recurring) or use baseband modems with proprietary encoding techniques on physical copper wires (assuming you could get them).

As it turned out, some of these encoding techniques were not as good as the others (but the equipment was relatively cheap, so the budget limits usually forced the decision). We had our own share of modem-related problems, but they were never as bad as what I've heard from one of my students: his modems would lose synchronization when transmitting a long string of zeroes over a regular synchronous serial interface; ping ip 1.2.3.4 size 1000 data 0000 would be enough to bring down the link.

Did you believe MPLS TE was a quality-of-service feature? Did someone persuade you it’s mandatory to run OSPF or IS-IS if you want to deploy MPLS TE? I’ve collected a few more myths like these two and explained the actual facts behind them in 10 MPLS traffic engineering myths and half truths published by ComputerWeekly..

In a previous post, I've described how you can turn your router into a TFTP server. As you can configure the router to serve any file residing on it, you can also pull startup and running configuration from it with TFTP, providing that you configure:

tftp-server nvram:startup-config
tftp-server system:running-config

Warning: Due to total lack of any security features in TFTP protocol, use this functionality only in lab environment.

I should probably write this one on April 1st, but maybe October 31st is not such a bad choice after all … if you configure no service prompt config, the configuration prompt is gone; when you enter the configuration mode with the configure terminal command, you get an empty line (like you did with Cisco software release 9.1 some 15 years ago). Similarly, you can disable command-line editing with the no editing line configuration command or terminal no editing exec-level command. If only there would be a way to disable the context-sensitive help :)

I did a few follow-up tests with the distribute-list in OSPF configuration command and stumbled across a few interesting facts (IOS release 12.4(15)T1 on a 3725 platform):

• Although the router allows you to configure distribute-list acl in interface, it does not work. Routes received through that interface (or having the interface as the next-hop) are not filtered.
• When you apply the distribute-list in command, the routing table is not changed. Clearing the IP routing table does not help, you have to clear ALL OSPF processes (including bringing down all OSPF adjacencies) with the clear ip ospf process command for the route filter to take effect.
• The same limitations don't apply in the other direction: when you remove the distribute-list in, SPF is triggered and the routes appear in the IP routing table automatically.
• The somewhat undocumented gateway option of the distribute-list in command works, but not quite as I would expect: the IP next hop, not the router-ID of the router advertising the IP prefix is matched by the prefix-list.

And, last but not least, I've lab-verified my previous claim: applying the distribute-list in on a transit router can result in a black hole, as the LSAs themselves are not filtered.